PaperlayerView exact revision on arXiv

Source-linked AI summary

Point-to-Point Communication in Integrated Satellite-Aerial Networks: State-of-the-art and Future Challenges

Nasir Saeed, Heba Almorad, Hayssam Dahrouj, Tareq Y. Al-Naffouri, Jeff S. Shamma, Mohamed-Slim Alouini

arXiv:2012.06182v1eess.SPcs.NIcs.SYeess.SY

TL;DR

Existing surveys focus on single non-terrestrial layers, while microwave RF links face spectrum constraints and FSO links are weather-sensitive and alignment-dependent. This paper surveys P2P links across satellite and HAP layers, covering RF and FSO options and satellite link-budget analyses.

  • Problem

    Existing surveys focus on single non-terrestrial layers, while microwave RF links face spectrum constraints and FSO links are weather-sensitive and alignment-dependent.

    Source: §I · p. 2, B. Contributions of our Paper · p. 3

  • Method

    The paper surveys P2P links across satellite and HAP layers, covering RF and FSO options and reviewing satellite link budgets and HAP link studies.

    Source: B. Contributions of our Paper · p. 3, §VI · p. 14

  • Results

    The survey presents link-budget analyses for RF- and FSO-based satellite links and reviews performance analysis studies of HAP networks.

    Source: §VI · p. 14

  • Takeaways & Limitations

    Spatial networks are described as essential enablers for serving remote areas and enhancing existing wireless-system capacity.

    Source: §I · p. 1

Abstract

from arXiv · show

This paper overviews point-to-point (P2P) links for integrated satellite-aerial networks, which are envisioned to be among the key enablers of the sixth-generation (6G) of wireless networks vision. The paper first outlines the unique characteristics of such integrated large-scale complex networks, often denoted by spatial networks, and focuses on two particular space-air infrastructures, namely, satellites networks and high-altitude platforms (HAPs). The paper then classifies the connecting P2P communications links as satellite-to-satellite links at the same layer (SSLL), satellite-to-satellite links at different layers (SSLD), and HAP-to-HAP links (HHL). The paper overviews each layer of such spatial networks separately, and highlights the possible natures of the connecting links (i.e., radio-frequency or free-space optics) with a dedicated overview to the existing link-budget results. The paper, afterwards, presents the prospective merit of realizing such an integrated satellite-HAP network towards providing broadband services in under-served and remote areas. Finally, the paper sheds light on several future research directions in the context of spatial networks, namely large-scale network optimization, intelligent offloading, smart platforms, energy efficiency, multiple access schemes, and distributed spatial networks.

I. INTRODUCTION

Spatial networks combine satellite and aerial layers to extend connectivity, but point-to-point links face trade-offs between RF capacity and FSO weather and alignment sensitivity. This survey situates its coverage of those links within prior reviews focused on particular network layers and topics.

I. INTRODUCTION

  • Almost half the world’s population remains unconnected, while projected growth beyond 50 billion connected devices is expected to burden terrestrial telecommunications infrastructure.The introduction frames new connectivity solutions as necessary to meet these demands.Source: §I · p. 1
  • Spatial networks connect nodes across altitude layers; multilayer architectures combine layer benefits, while multihop relaying can reduce propagation delay and improve data rate,.Nodes include satellites, HAPs, tethered balloons, and UAVs; links can be intra-layer or inter-layer and use RF or FSO.Source: §I · pp. 1–2
  • CDL microwave links are limited to 274 Mbps to 3 Gbps, whereas FSO can reach 100 Gbps but is weather-sensitive and requires precise alignment –.The FSO rate is reported with wavelength-division multiplexing.Source: §I · p. 2
  • Hybrid FSO/RF systems can switch to RF when weather obscures line of sight, but scheduling, scalability, and quality-of-service constraints remain,.Source: §I · p. 3

A. Related Review Articles

  • Prior HAP reviews address P2P channel modeling and FSO acquisition, tracking, and pointing, while another surveys rural HAP coverage and capacity,,.Source: A. Related Review Articles · p. 3
  • Satellite-oriented reviews cover inter-satellite design, delay-tolerant networking, and FSO; spans SN layers but focuses mainly on satellites,,,.Source: A. Related Review Articles · p. 3

B. Contributions of our Paper

The paper surveys P2P links across multiple spatial-network layers, including RF and FSO options, and considers integrated satellite-HAP networks for broadband in underserved areas. It then organizes the review by network layer and introduces satellite constellation, relaying, and topology considerations.

B. Contributions of our Paper

  • Unlike surveys limited to one non-terrestrial layer, this paper covers multilayer spatial-network P2P links, RF and FSO connections, integrated satellite-HAP broadband prospects, and six future research directions.The directions are large-scale optimization, intelligent offloading, smart platforms, energy efficiency, multiple access, and distributed spatial networks.Source: B. Contributions of our Paper · p. 3

C. Paper Organization

  • The review proceeds from satellite P2P links and their RF/FSO link budgets to HAP links, integrated satellite-HAP networks, and future research directions.Source: C. Paper Organization · p. 3

II. P2P LINKS IN SATELLITE NETWORKS

  • Satellite altitude, constellation, and operating frequency shape coverage, delay, and Doppler: higher orbits cover more area, while lower-orbit paths are shorter and satellites move faster [49].Highly elliptical orbits can be designed to increase dwell time over selected regions [49].Source: §II · p. 4
  • Satellite relaying uses amplify-and-forward, which forwards an amplified signal, or decode-and-forward, which decodes and processes the signal to mitigate interference.Source: §II · p. 4
  • Star, mesh, and line topologies differ in connectivity: star reduces failure chances, while mesh offers more flexibility and lower latency at greater complexity [49],.Star connects satellites to a central node, mesh directly connects all satellites, and line connects neighbors.Source: §II · p. 4

A. Satellite-to-Satellite Links at Same Layer (SSLL)

Same-layer satellite-to-satellite links (SSLL) are mainly simplex links whose path length is the line-of-sight distance. The section characterizes RF link budgets and illustrates how distance and frequency affect link performance.

A. Satellite-to-Satellite Links at Same Layer (SSLL)

  • SSLL are mainly simplex links, measure path length by the satellites’ line-of-sight distance, and can use either RF or free-space optical technology.The section notes that SSLL have been studied by NASA, ESA, and DARPA for over a decade.Source: A. Satellite-to-Satellite Links at… · pp. 4–5

1) RF Link Budget:

  • For RF SSLL, coherent BPSK needs less power for a given throughput and BER, while QPSK doubles BPSK bandwidth but can suffer phase distortion.Differential PSK is described as a way to improve spectral efficiency while trading off power requirements and spectral efficiency.Source: 1) RF Link Budget · p. 5
  • The RF link budget uses transmit power, satellite distance, data rate, wavelength, and transmit-aperture diameter, with antenna gain and path loss informing received power.The analysis assumes a given modulation scheme, no coding, and isotropic transmit-antenna radiation.Source: 1) RF Link Budget · pp. 5–6, Table III · p. 6
  • At 22.5 GHz, a 100 km RF SSLL receives -125 dBm; energy-per-bit to noise spectral-density values fall with distance and are higher at lower frequencies.At 5 km, the reported values are -2 dBm at 60 GHz and 19 dBm at 5.8 GHz; at 100 km, they are -48 dBm and -28 dBm, respectively.Source: Figure 3 · p. 6, Figure 4 · p. 6, 1) RF Link Budget · p. 6

2) Optical Link Budget:

The optical link-budget discussion describes a line-of-sight satellite link and an LED-to-photodetector model. It considers optical-link performance alongside the pointing and acquisition constraints of high-speed FSO.

2) Optical Link Budget:

  • FSO offers higher data rates and greater usable bandwidth than RF, but high-speed optical links need directive beams and face acquisition, tracking, and pointing challenges that restrict short-range SSLL.Photon-counting detector arrays are noted as one possible way to improve signal acquisition for long-range FSO.Source: 2) Optical Link Budget · pp. 6–7
  • Optical SSLL modulation choices trade power efficiency, bandwidth efficiency, implementation complexity, and threshold requirements across OOK-NRZ, M-PPM, SIM, homodyne BPSK, and DPSK.The passage describes OOK-NRZ as widely adopted, M-PPM as power-efficient, SIM as more bandwidth-efficient than M-PPM, and DPSK as reducing power requirements while increasing design complexity.Source: 2) Optical Link Budget · p. 7
  • The link-budget model assumes LED transmitters, photodetector receivers, and OOK-NRZ modulation, and describes the line-of-sight channel using satellite distance and incidence and viewing angles.The model also defines Lambertian emission and concentrator-related parameters.Source: 2) Optical Link Budget · p. 7
  • The numerical illustration reports that received optical power decreases as satellite distance increases, while a smaller concentrator field-of-view angle yields slightly more received power.The setup considers LEO satellites and plots received power against concentrator field-of-view semi-angle.Source: 2) Optical Link Budget · p. 7, Figure 6 · p. 8

B. Satellite-to-Satellite Links at Different Layers (SSLD)

The section motivates multilayer satellite links as an alternative to restrictions associated with single-layer networks. It reviews architectures, implementations, and optical-link considerations for links between satellite layers.

B. Satellite-to-Satellite Links at Different Layers (SSLD)

  • Single-layer satellite networks can face accumulated multi-hop delay, low stability from planar topologies, and congestion associated with repeated handovers and routing changes.The paper says these restrictions make establishing and maintaining a single-layer network harder.Source: B. Satellite-to-Satellite Links at… · p. 8
  • Reviewed multilayer designs include MEO–LEO networks combining within-layer SSLL with inter-layer SSLD, and a GEO–MEO–LEO architecture proposed to improve capacity, reliability, and coverage.One MEO–LEO design routes short-distance services through SSLL and relays long-distance traffic via MEO satellites using SSLD.Source: B. Satellite-to-Satellite Links at… · p. 8
  • Projects including OICETS, ARTEMIS, and Alphasat TDP1 Sentinel-1A use optical links to relay data between satellites at different altitudes.The section also reviews optical-system proposals, routing, and topology-analysis work for multilayer satellite networks.Source: B. Satellite-to-Satellite Links at… · p. 8
  • FSO is presented as promising for inter-layer satellite links because its radiated beam is not affected by turbulence, while reliable, stable links still require efficient acquisition, tracking, and pointing.Source: B. Satellite-to-Satellite Links at… · p. 9

III. P2P LINKS IN HAP NETWORKS

HAPs operate around 20 km above Earth and can remain quasi-static, supporting connectivity across an operating area. Interconnected HAPs are presented as a potential way to provide broad-region internet access and remote sensing.

III. P2P LINKS IN HAP NETWORKS

  • At around 20 km altitude, quasi-static HAPs can provide ubiquitous connectivity in their operating area; interconnected HAPs are a potential solution for broad-region internet access and remote sensing.The passage cites rural and disaster-affected regions as connectivity applications and describes HAP backhauling and extended coverage as motivations for inter-HAP links.Source: §III · p. 9

A. HAP-to-HAP Links (HHL)

Inter-HAP links favor FSO for high-speed communication, though weather, turbulence, geometric spreading, and pointing errors affect performance. The section also reviews RF alternatives and channel models for these links.

A. HAP-to-HAP Links (HHL)

  • RF inter-HAP and backhaul links face high bandwidth and transmit-power requirements over long ranges, and higher RF bands are severely affected by rain attenuation.Among cited studies, DVB-S2 backhauling showed lower BER than WiMAX at lower SNR.Source: A. HAP-to-HAP Links (HHL) · p. 9
  • Inter-HAP links prefer FSO for high-speed communication, while HAP-to-ground links mainly use RF; HAPs operate above clouds, reducing FSO links’ weather exposure.A proposed 500 km inter-HAP FSO link at 20 km altitude achieved 384 Mbps with 10−6 BER.Source: A. HAP-to-HAP Links (HHL) · p. 9
  • FSO link-budget analysis accounts for geometric spreading, turbulence, temperature inhomogeneity, and pointing error; the Hufnagel–Valley model estimates altitude-varying refractive-index structure using wind speed ν and constant K.Geometrical loss depends on path length, receiver-aperture radius, and beam-divergence angle.Source: A. HAP-to-HAP Links (HHL) · p. 9
  • Pointing errors can cause link failure or reduce received power and raise BER; proposed mitigation includes a wider receiver FoV, multiple-beam transmissions, hybrid RF/FSO, and adaptive optics.A hybrid triple-hop RF-FSO-RF design uses FSO between HAPs in good weather and RF in bad weather or without LoS.Source: A. HAP-to-HAP Links (HHL) · pp. 9–11
  • The gamma-gamma channel model fits both weak and strong turbulence, while log-normal and exponential models are less consistent outside their respective regimes.The section gives the irradiance probability density and a scintillation-index expression using the turbulence parameters ᾱ and β̄.Source: A. HAP-to-HAP Links (HHL) · p. 10
  • For HAP-to-ground FSO backhaul under turbulence, one cited study achieved 1.25 Gbps with BER below 10−9.Other reviewed HAP-to-ground studies examine coverage, capacity, interference, and channel conditions –.Source: A. HAP-to-HAP Links (HHL) · p. 11

B. Handover between HAPs

HAP movement and airflow can alter ground footprints, making handover schemes relevant for maintaining user links. Prior work considers cell traffic, platform direction, modulation, cooperation, and prediction in handover decisions.

B. Handover between HAPs

  • Airflow can change a HAP’s ground footprint, and handover transfers users’ communication links between cells to avoid channel instability, especially when extended-coverage cells differ greatly in size [48].Source: B. Handover between HAPs · p. 11
  • HAP handover studies address traffic differences during transfer, platform travel direction, adaptive modulation, cell cooperation, platform movement, and prediction-based decisions, –.One proposed adaptive soft handover algorithm uses platform downlink output power and individual base stations.Source: B. Handover between HAPs · p. 11

IV. INTEGRATED SATELLITE-HAP COMMUNICATION NETWORKS

Integrated satellite-HAP-LAP networks are presented as a way to improve the coverage, reliability, and scalability of 6G systems, including broadband access in underserved areas. The section surveys architectures combining spatial and terrestrial components and their outstanding challenges.

IV. INTEGRATED SATELLITE-HAP COMMUNICATION NETWORKS

  • Satellite networks’ large footprints and ubiquitous remote-area coverage make them a possible 6G broadband enabler, while integrating satellites, HAPs, and LAPs may improve coverage, reliability, and scalability –.The envisioned nodes at same or different altitudes can connect through RF or optical links, with satellites providing RF/optical backhaul for HAPs and LAPs.Source: §IV · p. 11
  • Proposed integrated spatial-terrestrial approaches include densified satellite, UAV, and ground-vehicle networks, SDN-based moving cells, coordinated LEO and terrestrial small cells, and cloud-based joint processing –.Reported challenges include interoperability, resource allocation, and network management in dynamic settings; one LEO approach requires ultra-dense deployment and omits HAPs and LAPs.Source: §IV · p. 11
  • The HAP-network architecture illustration depicts P2P HAP-to-HAP links alongside backhauling links.Source: Figure 8 · p. 12
  • A heterogeneous spatial network connects satellite, HAP, and LAP backbone entities by laser links and provides user access through microwave links.For HAP-based FSO relaying between ground and LEO satellites, a cited study reports a 28 dB power-gain increase at BER of 10−9.Source: §IV · p. 12

V. FUTURE RESEARCH DIRECTIONS

Research on complex, large-scale spatial networks remains at an initial stage, and the section identifies further investigation as necessary.

V. FUTURE RESEARCH DIRECTIONS

  • Because studies of complex, large-scale spatial networks are still at an initial stage, the paper identifies multiple problems as needing further investigation.Source: §V · p. 12

A. Network Optimization

Integrated spatial-network optimization must account for heterogeneous node characteristics, while coordinated offloading must balance the trade-offs among terrestrial and satellite links. The paper identifies advanced joint optimization and better offloading decisions as open research needs.

A. Network Optimization

  • Spatial-network optimization must account for diverse layer characteristics and address cost, mobility, energy and spectrum efficiency, security, traffic offloading, and user experience.Existing optimization work for spatial networks remains relatively limited, motivating advanced joint techniques.Source: A. Network Optimization · p. 12

B. Intelligent Offloading

  • Coordinated traffic offloading is more complex in integrated spatial-terrestrial networks than in standalone networks, and decisions can weigh link cost, reliability, and capacity.Satellite links offer wider footprints and continued services but have large latency and lower QoE than terrestrial links.Source: B. Intelligent Offloading · p. 13
  • A latency-aware scheme routes URLLC traffic to terrestrial backhaul and eMBB traffic to satellites, whose traffic need not always meet stringent delay requirements.Source: B. Intelligent Offloading · p. 13
  • SDN can support intelligent traffic offloading in integrated spatial-terrestrial networks by separating data and network planes.Source: B. Intelligent Offloading · p. 13

C. Smart Platforms

The paper outlines research directions for smart surfaces, energy efficiency, multiple access, and distributed spatial networks. It concludes that P2P links merit study amid spatial-network constraints, surveying link technologies and future research needs.

C. Smart Platforms

  • Smart surfaces are flexible metamaterials that passively or actively reflect received signals; their spatial-platform applications are promising but remain an early-stage research area.Proposed benefits include energy efficiency, improved coverage, and lower system complexity.Source: C. Smart Platforms · p. 13

D. Energy Efficiency

  • Spatial platforms have limited power and face radiation, environmental, and propagation effects, motivating low-PAPR amplifiers, non-orthogonal waveforms, SDN/NFV, and energy harvesting.Splitting control and data planes with SDN has been reported to yield significant energy gain for integrated spatial-terrestrial networks.Source: D. Energy Efficiency · p. 13

E. Novel Multiple Access Schemes

  • RSMA can be applied horizontally at one layer or vertically across layers, but its use in integrated spatial-terrestrial networks remains uninvestigated [162].The passage notes that SDMA and NOMA gains are limited because they depend on environmental conditions.Source: E. Novel Multiple Access Schemes · p. 13

F. Distributed Spatial Networks

  • Real-time distributed optimization should model changing network conditions and platform positions, including dynamic UAV placement, satellite constellations, and HAP placement.Satellites move in predetermined constellations, HAPs are relatively stationary, and LAPs can adjust position to ground demand and network heterogeneity [163].Source: F. Distributed Spatial Networks · p. 13

VI. CONCLUSIONS

  • High latency, constrained resources, mobility, and intermittent links distinguish spatial networks from terrestrial networks and make inter-layer P2P links important to study.Source: §VI · p. 13
  • The paper surveys P2P links across spatial-network layers, including RF and FSO satellite and HAP links, link-budget analyses, integrated networks, and future research directions.The authors describe it as the first survey of P2P links for multilayer spatial networks in the context of 6G large-scale complex networks.Source: §VI · p. 14
Loading 2012.06182v1…